Graduate Theses, Dissertations, and Problem Reports
2015
Hierarchically Porous Carbon Materials and LiMn2O 4 Electrodes for Electrochemical Supercapacitors
Shimeng Hao
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Electrodes for Electrochemical Supercapacitors
Shimeng Hao
Thesis submitted to the
Benjamin M. Statler College of Engineering and Mineral Resources
at West Virginia University
in partial fulfillment of the requirements for the degree of
Master of Science
In
Mechanical Engineering
Nianqiang Wu, Ph.D., Chair
Terence Musho, Ph.D.
Feng Yang, Ph.D.
Ayyakkannu Manivannan, Ph.D.
Department of Mechanical and Aerospace Engineering
Morgantown, West Virginia
2015
Keywords: Supercapacitor, energy, porous carbon, LiMn2O4, hierarchically porous structure, lignin
Copyright 2015 [Shimeng Hao]
Abstract
Hierarchically Porous Carbon Materials and LiMn2O4 Electrodes for
Electrochemical Supercapacitors
Shimeng Hao
Increasing energy density of electrochemical capacitors (ECs) is crucial for their applications in energy storage devices requiring short peak power pulses as well as long- term operation. ECs are operated via two primary charge mechanisms, that is, the electrochemical double-layer capacitance and the pseudocapacitance. In the thesis, the carbon materials and LiMn2O4, which generate double-layer capacitance and pseudocapacitance, respectively, have been investigated. The effects of specific surface area, pore structure and surface functionality on the energy storage performance of ECS have been studied. Micro-porous (<2 nm) carbon with pores inaccessible to the solvated ions may limit the ion diffusion, resulting in a low rate capability. Hence this work attempts to generate hierarchical macropores/mesopores/micropores in the electrode material. Flexible, self-sustained and hierarchical porous carbon nanofibers (CNFs) are fabricated using terephthalic acid as the sacrificial agent. After sublimation and carbonization, the electrospun mat is converted to a hierarchical porous carbon framework. The high specific capacitance and good rate capability are associated with the unique hierarchical porous structure of the as-prepared CNFs. Both the outer fiber surface and inner porous structure can be accessible for charge accumulation through pores on the surface. Hierarchical macropores/mesopores in the fiber also help accelerate the ion-diffusion into inner micropores. Besides fossil resources, renewable biomass has also been explored as the source material for supercapacitors in the present work. Lignin, the major aromatic constituent of plant and woods, is utilized as the carbon precursor to prepare the mesoporous lignin- char. The lignin-derived carbon is prepared by taking an advantage of the organic-organic self-assembly method, which allows the direct formation of mesoporous polymer composite from carbon precursor and block copolymer, and conversion to porous carbon by carbonization. Hierarchically porous carbon (HPC) with pores at different scales has been obtained after alkali activation. The experimental results show that the appropriate pore size distribution can ensure high power density and high energy density due to the short diffusion distance and the minimized electric resistance. Utilization of biomass as the source materials for supercapacitors will reduce the costs for fabrication of energy storage devices.
Developing asymmetric supercapacitors is an alternative effective way to obtain high energy density for an enlarged potential window and additional pseudocapacitance. LiMn2O4 nanoparticles have been fabricated with a facile and cost-effective method using carbon black as the template. The spinel structured LiMn2O4 exhibits a high specific capacitance in a three-electrode system. An asymmetric supercapacitor has been made with the as-prepared LiMn2O4 nanoparticle as the cathode and the commercial activated carbon as the anode in a Li2SO4 aqueous solution. The asymmetric supercapacitor shows a good energy capacity and excellent cycling stability.
Acknowledgements
I would like to thank my supervisor Professor Nianqiang (Nick) Wu for his intellectual guidance and invaluable instructions in my 3-year studies for Master of
Science degree in West Virginia University.
I would also like to thank my committee members Dr. Terence Musho, Dr. Feng
Yang and Dr. Ayyakkannu Manivannan for their advice and help on my thesis.
In addition, I would like to express my thanks to Dr. Weiqiang Ding, Dr. James
Poston for their support of materials characterization with XPS and XRD.
Special thanks to Dr. Jiangtian Li for his valuable advice and help on my research.
I am also grateful to Peng Zheng, Joeseph Bright, Sujan Phani Kumar Kasani, Dr. Scott
Cushing, Savan Suri, Dr. Jianliang Cao, Dr. Yan Wang, Dr. Hualei Zhou, Dr. Xuefei
Gao,.Yang He for their help in academic field and everyday life.
Finally, I wish to express my sincere appreciation to my friends and family for their encouragement and support throughout all the time.
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Table of Contents
Chapter 1 Introduction...... 1 1.1 Background ...... 1 1.2 Motivation ...... 2 1.3 Significance...... 6 References ...... 7 Chapter 2 Literature Review ...... 9 2.1 Overview of supercapacitors ...... 9 2.2 Principles of supercapacitors ...... 11 2.3 Two mechanisms of supercapacitor ...... 14 2.3.1 Electrochemical double-layer capacitors ...... 14 2.3.2 Pseudocapactitors ...... 16 2.3.3 Hybrid capacitors ...... 17 2.4 Electrode materials ...... 17 2.4.1 Carbon ...... 18 2.4.2 Pseudocapacitive materials ...... 20 2.5 Electrolyte ...... 21 2.6 Evaluation of supercapacitor performance ...... 22 2.6.1 Cyclic Voltammetry ...... 22 2.6.2 Constant Current Charge-Discharge Test ...... 22 2.6.3 Electrochemical Impedance Spectroscopy ...... 23 2.6.4 Durability Test ...... 23 References ...... 24 Chapter 3 Hierarchically Porous Carbon Nanofiber as Flexible Electrode for Symmetric Supercapacitor ...... 29 3.1 Background and introduction ...... 29 3.2 Experimental section ...... 31 3.2.1 Electrode preparation ...... 31 3.2.2 Characterization of the carbon fiber ...... 31 3.2.3 Electrochemical testing ...... 32 3.3 Results and discussion ...... 33 3.4 Conclusions ...... 43 References ...... 43
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Chapter 4 Asymmetric Supercapacitors from Nano-architectured LiMn2O4 // Activated Carbon Electrodes ...... 47 4.1 Background and introduction ...... 47 4.2 Experimental section ...... 48 4.2.1 Electrode preparation ...... 48 4.2.2 Characterization ...... 49 4.2.3 Electrochemical characterization ...... 49 4.3 Results and discussion ...... 50 4.4 Conclusions ...... 56 References ...... 56 Chapter 5 Lignin-derived Hierarchically Porous Carbon Prepared by a Self- assembly Method for Electrochemical Supercapacitor ...... 60 5.1 Introduction ...... 60 5.2 Experimental section ...... 62 5.2.1 Synthesis of lignin-derived hierarchically porous carbon ...... 62 5.2.2 Material characterization ...... 63 5.2.3 Electrochemical testing ...... 63 5.3 Results and discussion ...... 64 5.4 Conclusions ...... 71 References ...... 71 Chapter 6 Conclusions ...... 73
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Chapter 1 Introduction
1.1 Background
Energy storage is to balance the supply and demand of energy and is becoming a
key factor in economic growth with the widespread application of electricity. Renewable
energy (most notably solar and wind) contributed 19% to the energy consumption and 22%
to the electricity generation in 2012 and 2013 based on REN21’s 2014 report [1].
However, the intermittent renewable energy sources limit continuous electricity power
supply and effective electric energy storage systems are in great demand to capture
excess energy during periods of low demand.
Principles of electrical energy storage are divided into two types: chemical energy
storage and capacitive energy storage. Batteries store energy in the form of chemical
reactants whereas electrochemical capacitors (ECs) store energy as charge. Although
batteries are ubiquitous in today’s potable electronic devices because of recent
improvements in engineering and chemistry, to meet the requirements of high power
recovery-supply or high charge-discharge cyclability in some application devices remains a challenge. For example, only energy storage devices with fast charging rate can capture
energy which is currently wasted in many repetitive processes such as braking in
automobiles and descending elevators. Rather than the phase and crystalline structure
changes caused by the faradaic charging-discharging in batteries, no major changes take
place when ECs store electrical charge. Therefore, the ECs systems can undergo a large
number of charging-discharging cycles (up to millions) with high stability and reliability.
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Furthermore, this storage mechanism is particularly adapted for applications which
require fast charging-discharging rate (within seconds).
The world market of batteries is estimated to be $95 billion while that of
supercapacitors is only $400 million in 2013 [2]. To fully harness ECs’ potential as
energy storage systems, it is essential to develop ECs with higher energy density and lower costs. [3].
1.2 Motivation
To satisfy the industrial demands and realize the full potential of ECs as electrical energy storage devices, new electrode materials/design should be proposed and fundamental understanding of the physical and chemical processes at the interface is required. The field of ECs has been strongly influenced by battery technology which is evident by the construction of similar electrode materials. Two active layers, one separator with electrolytes are the basic component elements of both the two kinds of energy storage systems. Carbon can be used as anodes, transition-metal oxides as cathodes, and sulfuric acid or acetonitrile can serve as electrolytes [4], which are much like the construction of batteries. However, electrode/electrolyte materials should be designed specific to ECs due to the different mechanisms between the two storage systems [5].
High costs and low-energy storage have been considered as the major obstacles for ECs to meet demands of energy storage device requiring short peak power pulses as well as long-term operation [6]. Natural precursors or biomass materials have been widely investigated in recent years as nanostructured activated carbon electrodes for
2
electrical double layer capacitors (EDLCs), motivated in large part by their low-cost, high
specific surface area, high electrical conductivity, high stability, environmental friendly
character and ease of synthesis and processing [7-10]. The total carbon mass of biomass
production is estimated to be 104.9 petagram (104.9×1015 g) per year [11]. The great
abundance of carbon sources makes biomass-derived carbon promising candidates for
low cost and mass-production of carbon-based EDLCs. Different physical and chemical
activation methods can lead to extensively developed porosity and high value of specific
surface area (~2400 m2g-1) [12]. However, highly micro-porous (<2 nm) carbons limits
the ion diffusion in case of smaller pores compared to the solvated ions, resulting in
lower capacitance at high-rate charge/discharge rate. A 3D hierarchical porous carbon
design is proposed and macro-pores (>50 nm), meso-pores (2~50 nm) and micro-pores
(<2 nm) are combined to achieve improved power density and energy density at high rate
[13], because the macro-pores can minimize the diffusion distances, the meso-pores offer
low-resistant ion transport pathways for the electrolyte ions whereas the micro-pores
contribute to the capacitance values [14,15]. Besides the consideration of pore-size
distribution, other factors such as surface functionality, hydrophilicity, electrical conductivity also play an important role in energy storage performance of carbon electrode [16]. Therefore, even if good performance is achieved with well-developed porosity, the other parameters are important considerations and worth further investigation to make this system applicable in energy storage.
Electrospinning- drawing submicrometer fibers from a liquid by an electrical charge-is a powerful technique for the fabrication of one-dimensional (1D) nanostructured fibers. Polyacrylonitrile is a widely-used polymer precursor for
3
electrospinning process due to its good electrospinnabilty. Upon the following
carbonization treatment, continuous carbon nanofibers (CNFs) are easily produced with
nano-scale diameter, large surface-to-volume ratio, large surface area and high conductivity. To further enhance the application of electrospun carbon fiber, optimizing composite precursor has been utilized to increase the surface area and porosity of carbon fibers by selective removal of one component in the composite [17], which provides a cost-effective synthetic route to control internal fiber structure (e.g. pore size distribution) in the porous carbon fiber. Different kinds of pores (micro-, meso-, macro-) can be generated with variations of precursor options. In addition, the electrospinning parameters and the carbonization temperature are responsible for the final carbon fiber diameter, surface morphology, the degree of graphitization and electrical conductivity, which affect the capacitance and rate capability of CNFs-based electrochemical capacitors. Another practical issue is that the CNFs fabricated after carbonization is usually brittle, and flexible CNFs which can be integrated into the wearable electrical devices are in great demand in industry. All the above challenges should be overcome before the industrial application of CNFs as supercapacitors.
Pseudocapacitor, besides EDLCs, is the other type of electrochemical capacitors.
Its operation is based on electrosorption, reduction-oxidation reactions, and intercalation processes instead of a Faradaic process. Therefore, it possesses intrinsically higher capacitance and energy density than EDLCs. However, the high cost (e.g. RuO2) and low
conductivity (e.g. MnO2) have limited its potential application [18,19]. Asymmetric hybrid capacitor, which combines Faradaic and non-Faradaic processes, is a promising system to exploit both the charge/discharge of capacitive double-layer and that of
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pseudocapacitive materials. EDLCs-type electrode may provide high power density while pseudocapacitve materials provide high energy density. Investigation into the novel and rational electrodes design may lead to electrochemical supercapacitors that store more energy at higher charge/discharge rate.
In this thesis, experiments are designed and conducted based on two different types of energy storage mechanisms in electrochemical capacitors: double layer charge capacitance and pseudocapacitance. Tailored nanostructured materials are developed to enable fast ion transport, high conductivity and high surface area. Apart from the scientific quest for active materials with high energy density and power density, flexible
EDLCs are also fabricated to realize its incorporation into wearable energy storage devices.
The thesis presents a brief introduction of the work involving the background and motivation. In chapter 2, a literature review is provided based on the progress of electrochemical supercapacitor in recent years. Following the introduction and review section, in chapter 3, we provide a facile route to fabrication of flexible carbon nanofibers with hierarchically porous structure via electrospinning. In chapter 4, LiMn2O4 nanoparticles were fabricated with a facile and cost-effective method by using carbon black as template. An asymmetric supercapacitor was made with the as-prepared
LiMn2O4 nanoparticle as the cathode and the commercial activated carbon as the anode working in Li2SO4 aqueous solution. In chapter 5, we utilized lignin as carbon precursor to prepare mesoporous lignin-char. Hierarchically porous carbon with pores at different scales were obtained after alkali activation. The effects of specific surface area and hierarchical porous nano-structure on the energy capacity are discussed.
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1.3 Significance
In the thesis, we present the fabrication processes of three types of active
materials for electrochemical capacitors. The unique hierarchical pore structure and nano-
structure exhibit high performance in energy storage. The results will not only be helpful
to the future design of advanced electrode materials and configurations, but also be
beneficial to better understand the physical and chemical processes at the
electrode/electrolyte interface. Ions in the electrolyte cross the pore space onto the
surface of carbon via diffusion, but too small or tortuous micropores may hinder the ion
transport. For the as-prepared carbon nanofibers, both the outer fiber surface and inner
pores can be accessible for charge accumulation through pores on the surface.
Hierarchical macropores/mesopores in the fiber are also beneficial to accelerate the ion-
diffusion into inner micropores. In the hierarchically porous carbon design, macro-pores,
meso-pores and micro-pores are combined to achieve improved power density and
energy density at the same time: the macro-pores can minimize the diffusion distance, the
meso-pores accelerates ion-transport whereas the micro-pores contributes to the
capacitance values. Cost is another consideration for electrochemical capacitors to meet demands of energy storage. The utilization of lignin, the major aromatic constituent of plant and woods, enables potential applications in low-cost energy storage devices. These electrodes materials are promising candidates for high-performance electrochemical capacitors.
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References
[1] REN21. Renewables 2014: Global Status Report. (2014).
[2] Dennis Zogbi, Paumanok Group. Supercapacitors the Myth, the Potential and the
Reality. (2013)
[3] Kötz, R., and M. Carlen. "Principles and applications of electrochemical capacitors."
Electrochimica Acta 45 (2000): 2483-2498.
[4] Pandolfo, A. G., and A. F. Hollenkamp. "Carbon properties and their role in supercapacitors." Journal of power sources 157 (2006): 11-27.
[5] Goodenough, J. B., H. D. Abruña, and M. V. Buchanan. "Basic Research Needs for
Electrical Energy Storage: Report of the Basic Energy Sciences Workshop on Electrical
Energy Storage, 2007 Apr 04. US Department of Energy."
[6] Zhang, Jiujun, et al., eds. Electrochemical technologies for energy storage and conversion. John Wiley & Sons, 2012.
[7] Kötz, R., and M. Carlen. "Principles and applications of electrochemical capacitors."
Electrochimica Acta 45 (2000): 2483-2498.
[8] Frackowiak, Elzbieta, and Francois Beguin. "Carbon materials for the electrochemical storage of energy in capacitors." Carbon 39 (2001): 937-950.
[9] Pandolfo, A. G., and A. F. Hollenkamp. "Carbon properties and their role in supercapacitors." Journal of power sources 157 (2006): 11-27.
[10] Wei, Lu, and Gleb Yushin. "Nanostructured activated carbons from natural precursors for electrical double layer capacitors." Nano Energy 1 (2012): 552-565.
[11] Field, Christopher B., et al. "Primary production of the biosphere: integrating terrestrial and oceanic components." Science 281 (1998): 237-240.
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[12] Zhu, Yanwu, et al. "Carbon-based supercapacitors produced by activation of
graphene." Science 332 (2011): 1537-1541.
[13] Wang, Da‐Wei, et al. "3D aperiodic hierarchical porous graphitic carbon material
for high‐rate electrochemical capacitive energy storage." Angewandte Chemie 120
(2008): 379-382.
[14] Xing, W., et al. "Superior electric double layer capacitors using ordered mesoporous
carbons." Carbon 44 (2006): 216-224.
[15] Chmiola, John, et al. "Anomalous increase in carbon capacitance at pore sizes less than 1 nanometer." Science 313 (2006): 1760-1763.
[16] Gogotsi, Yury, ed. Nanomaterials handbook. CRC press, 2006.
[17] Huang, Zheng-Ming, et al. "A review on polymer nanofibers by electrospinning and their applications in nanocomposites." Composites science and technology 63 (2003):
2223-2253.
[18] Abruña, Héctor D., Yasuyuki Kiya, and Jay C. Henderson. "Batteries and electrochemical capacitors." Phys. Today 61 (2008): 43-47.
[19] Bélanger, Daniel, L. Brousse, and Jeffrey W. Long. "Manganese oxides: battery
materials make the leap to electrochemical capacitors." The Electrochemical Society
Interface 17.1 (2008): 49.
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Chapter 2 Literature Review
2.1 Overview of supercapacitors
In 1853, Helmholtz showed that electrical double-layer (DL) could be used to
store charge based on the fact that charged electrodes immersed into electrolyte repel the
co-ions and attract counterions in the interface [1]. The first patent on porous carbon
electrodes in sulfuric acid electrolyte to store electrical energy was described in 1957 by
Becker for General Electric [2]. It was believed that high specific capacitances (around 5-
50 µF cm-2) could be obtained for carbons with high surface area in molten salts or aqueous electrolytes [3]. NEC developed aqueous electrolyte/activated carbon capacitors as “supercapacitors” in 1971 by licensing the technology of SOHIO [4]. In the following two decades, Panasonic and ELNA marketed the first generation of EDLCs which were mainly used for low current applications such as memory backup devices due to relatively high internal resistance [4,5]. Supercapacitors with low internal resistance were developed, which were used for the U.S. military applications with the Pinnacle Research
Institute (PRI) in 1982 [6].
Inspired by the work on conventional electrochemical battery, researchers studied another kind of supercapacitors (pseudopcapcitors) working in a different mechanism.
Pseudocapacitors store energy through electrosorption, redox reactions, and intercalation processes rather than an electrostatical double-layer charging process in the EDLCs [7-9].
By deposition onto conductive metallic substrates, conducting polymers and transitional- metal oxides (ruthenium oxide, nickel oxide, iridium dioxide) exhibited a higher energy density than EDLCs.
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In 2007, Department of Energy assessed the technologies on electrical storage and specifically showed the increasing potential of electrochemical capacitors in hybrid electric vehicle (HEV), portable devices and residential application [10]. Worldwide sales
of supercapacitor reached $400 million in 2010 and it is estimated to be $3.0 billion by
2016 according to Nanomarkets [11]. A number of companies including Ness, Panosonic,
Maxwell, EPCOS and ESMA have successfully marketed commercial ECs. Although
being able to deliver higher power per unit mass (Ragone plot, Figure 2-1), ECs store
lower energy than batteries which are widely used in a large range of electrical
applications. So far, the low energy density of ECs has been considered the main
challenge and a variety of electrodes have been developed to improve the energy storage.
Among them, activated carbons [12], carbon aerogels [13], carbon nanotubes [14],
graphenes [15] and carbon nanofibers [16] were investigated as ECs electrodes due to
their high electrical conductivity, stable physiochemical properties and versatility of
nano-structures. Recently, different kinds of porous structures (micro-, meso-, macro-)
have exhibited potential in improving the capacitance by either raising specific surface
area or minimizing ion transport resistance [17,18]. The accessible submicrometer pores
in the hierarchical porous structures may place an important role in enhancing the energy
density of porous carbons, especially those templated carbon materials.
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Figure 2-1 Ragone plot of energy storage and conversion devices
Many researches focus on the design of composite electrodes or asymmetric
hybrid capacitors, which can mitigate the disadvantages of EDLCs and pseducapacitors
to reach higher energy density. Companies such as Fuji Heavy Industry, ESMA and
CSIRO are also developing asymmetric hybrid capacitors for commercial devices [19-21].
Fundamental understanding and rational design of ECs will lead to a significant
increase of energy density while maintaining the feature of high power density. The study upon tailored porous structure, surface functionality, electrodes design is desirable to convert the potential of ECs into applications in energy storage field.
2.2 Principles of supercapacitors
Conventional capacitors consist of two electrodes separated by one insulating dielectric. When a voltage is applied to the system, charges of opposite polarity accumulate on the surfaces. Thus energy is stored in the form of electric field. The schematic of capacitors is illustrated in Figure 2-2.
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Figure 2-2 Schematic of a conventional capacitor
The capacitance of an ideal capacitor is a constant and it is defined as the ratio of
accumulated charge Q to the applied voltage V.